424 A. A. W. HUBRECHT. 
the outer surface of the body—which do arise from the epiblast; 
and afterwards become closely connected with the central 
nervous apparatus, would thus seem to be eminently sensory 
epithelia. It may be argued that it would indeed be much less 
comprehensible that a sensory epithelium should, by a secondary 
process, come to arise out of mesoblast cells than that the 
central apparatus should do so. 
One word more about the posterior brain-lobes (side organs 
auct.) and their central cavity. Larlier investigators have 
come to the following conclusions concerning them. Biitschli, 
who examined Pilidium larve (16) (of another species of 
Nemertea than our Lineus), describes these organs as epiblastic 
invaginations. He moreover mentions the presence of two 
diverticula of the cesophagus : a phenomenon wholly divergent 
from what was described above. Nevertheless a similar 
arrangement to that described for Pilidium by Bitschli is 
noticed in our Lineus at a late stage of development. These 
anterior diverticula of the cesophagus (l.c. (30), fig. 87) 
could, however, be demonstrated to be in no relation what- 
ever with the posterior brain-lobes. 
Metschnikoff (14), who also studied the Pilidium larva, 
regards the “‘ side organs” as esophageal diverticula. Barrois 
(21) similarly gives a detailed description in his account of the 
development of our identical Lineus obscurus of the origin 
of two lateral diverticula of the csophagus, which remain 
connected with it during a certain period by a special peduncle, 
figured in his plates, and disappearing later on, when the com- 
munication with the csophagus is finally given up. Barrois 
tells us that these diverticula become the side organs 
of the adult. 
It is a considerable difference on an important point be- 
tween my own observations above recorded and those of 
Barrois, that this investigator, together with Metschnikoff, 
looks upon the oesophagus as the starting-point for these 
diverticula. His account of the prolonged connection be- 
tween cesophagus and side organs is not in accordance with 
the real facts, which have taught us that the origin of the 
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